I'm using Objective-C language. But I don't know how to use c with Objective-C.
Ex) This is Function method.
- ( ?? ) function{
unsigned int first = ..
unsigned int second = ..
unsigned int third = ..
int infoData = {first,second,third};
return infoData;
}
How to fill in parenthesis.
I don't use NSArray.
Please help me.
the answer is the same as it is in C. Objective-C is a strict superset of C.
Assuming you declared int[] infoData you could make the return int*, but you're still going to have problems because the array is allocated on the function's stack. You'll need to dynamically allocate space for it just like you would in C.
(You cannot use int[] as a return type)
The code below will compile, but gcc will warn about returning the address of a function local variable.
#interface test
- (int*) function;
#end
#implementation test
- (int*) function{
unsigned int first = 0;
unsigned int second = 1;
unsigned int third = 2;
int infoData[] = {first,second,third};
return infoData;
}
#end
Related
I'm passing an array of structs from Swift to a C function. The struct looks like this:
struct Struct {
int a;
float b;
float c;
const char* d;
const char* e;
const char* f;
const char* g;
int h[4];
};
Function signature of the C function:
void test(struct Struct* structs);
Weirdly, when I print d in the C function, it's often something different than what I set it to in the Swift code: usually an empty string or some garbage. When I set d to a very long string, it works correctly. The other strings are passed correctly too. Is that some struct alignment issue?
As #MartinR suggested, when I pass the string to the struct's constructor, Swift creates a temporary C char array on the stack, copies the string's data into it and passes its pointer to the constructor. Immediately after that, the char array is no longer valid. Here's an example code demonstrating this:
let s = Struct(string: "string")
print(s.string) // Prints "string"
print("lol") // Overwrite the stack
print(s.string) // Prints "lol"
See https://stackoverflow.com/a/40121697/243225 for possible solutions.
Here is the method declaration midway in Apple's documentation:
Learning Objective-C: A Primer
- (void)insertObject:(id) anObject atIndex:(NSUInteger) index
Why is there no * right after NSUInteger. I thought all objects were pointer types and all strongly typed pointers had to have a * character after it.
NSUInteger is not an object type, it is a typedef to unsigned int.
The only reason that you would actually want to use a * in this context would be if you wanted to get the address of an int and store something in it. (Some libraries do this with error messaging). An example of this:
-(void) methodName: (NSUInteger *) anInt {
*anInt = 5;
}
NSUInteger a;
[obj methodName: &a]; //a is now 5
I confused when i want to take single pointer and when should i take double pointer?
In following structure what exactly did?
struct objc_class {
Class isa;
Class super_class;
const char *name;
long version;
long info;
long instance_size;
struct objc_ivar_list *ivars;
struct objc_method_list **methodLists;
struct objc_cache *cache;
struct objc_protocol_list *protocols;
};
Why we use the methodLists double pointer?
Edited
int sqlite3_get_table(
sqlite3 *db,
const char *zSql,
char ***pazResult,
int *pnRow,
int *pnColumn,
char **pzErrmsg
);
In above scenario what will be meaning of triple pointer char ***pazResult?
Well, in C at least, double-pointers are commonly used for 2D arrays. The most common 2D array is probably an array of C strings (char*'s). Double pointers are also sometimes employed to pass pointers to functions by reference, but this is unlikely to be the use in the code sample you posted.
According to the name methodLists I would guess that this is an array of lists. A (linked) list in C is commonly represented by a pointer to a node, which objc_method_list could be. An array of such lists is then implemented with a double pointer.
It's probably not the case in the code that you referenced, but you also need a double pointer any time you want to pass a pointer to a function and have changes to that pointer be reflected outside the scope of that function.
For example, if you were trying to rewrite the strcpy function so that the user did not have to allocate memory for the source string, you might try something like the following:
void MyStrcpy(char* dst, char* src){
dst = (char*)malloc(sizeof(char)*(strlen(src)+1));
for(int i=0;i<=strlen(src);i++)
dst[i] = src[i];
printf("src: %s ", src);
printf("dst: %s\n\n", dst);
}
If you were then to call that function,
int main() {
char *foo = "foo";
char *newPtr;
MyStrcpy(newPtr, foo);
printf("foo: %s ", foo);
printf("new: %s\n", newPtr);
}
your output would be as follows:
src: foo
dst: foo
foo: foo
new:
You might also get a seg fault when trying to print newPtr, depending your system. The reason for this behavior is the exact same as the reason you wouldn't expect a change to an int that was passed by value to a function to be reflected outside of that function: what you are passing to MyStrcpy is simply the memory address that newPtr references. When you malloc the space for dst inside the function, you are changing the address dst points to. This change will not be reflected outside of the scope of MyStrcpy!
Instead, if you wanted newPtr to point to the new allocated chunk of memory, you need to have dst be a pointer to a pointer, a char **.
void MyStrcpy(char** dst, char* src){
*dst = (char*)malloc(sizeof(char)*(strlen(src)+1));
for(int i=0;i<=strlen(src);i++)
(*dst)[i] = src[i];
printf("src: %s ", src);
printf("dst: %s\n\n", *dst);
}
Now, if you were to call that function:
int main() {
char *foo = "foo";
char *newPtr;
MyStrcpy(&newPtr, foo);
printf("foo: %s ", foo);
printf("new: %s\n", newPtr);
}
You would get your expected output:
src: foo
dst: foo
foo: foo
new: foo
Hope that helps!
See also these questions:
What is double star?
Why does NSError need double indirection? (pointer to a pointer)
In the most general case a double pointer is a pointer to a list of pointers.
In general pointer is used to hold the address of another variable. What if we need to hold the address of pointer ,in that case we use double pointer. When we want to hold the address of double pointer we use triple pointer.
I'm having trouble creating this c struct in objective c.
typedef struct huffman_node_tag
{
unsigned char isLeaf;
unsigned long count;
struct huffman_node_tag *parent;
union
{
struct
{
struct huffman_node_tag *zero, *one;
};
unsigned char symbol;
};
} huffman_node;
I'm getting this warning at the end of the union type and the end of the struct type above the "unsigned char symbol variable"
warning: declaration does not declare anything
And then when i do something like this:
huffman_node *p = (huffman_node*)malloc(sizeof(huffman_node));
p->zero = zero;
I get this compilation error:
error: 'huffman_node' has no member named 'zero'
Why does this not work? Did i set this up incorrectly? Has anyone experienced this before?
typedef struct huffman_node_tag
{
unsigned char isLeaf;
unsigned long count;
struct huffman_node_tag *parent;
union
{
struct
{
struct huffman_node_tag *zero, *one;
}; // problematic here!
unsigned char symbol;
}; // another problem here!
} huffman_node;
Depending on the C dialect/compiler that is being used to interpret the code, you may not be allowed to declare a struct or union without a name. Try giving them names and see what happens. Alternatively, you may want to try and change the C dialect you are using.
As far as I know anonymous unions are not part of C, but are a compiler extension. So strictly your given struct definition is not valid C. Consequently it seems that objective C does not supports this extension.
You need to include the header for the C library you are using.
You shouldn't have to do much else than that, as Objective C, unlike C++, is a strict superset of C.
I am trying to convert a unsigned char* to int * on in Objective-C on the iPhone. Is there any API that can help with the conversion?
Here's my best attempt:
-(BOOL)MyFunc:Version:(int *)nVer
{
unsigned char * uszVerSize;
//trying to assign string to int
nVer = uszVerSize[0] ;
}
Dear Lord, I think you have bigger problems than the one stated above.
You need to convert the chars to an int and return that.
return [[NSNumber numberWithUnsignedChar:uszVerSize] intValue];
You should also learn about pointers and how ints and chars differ in memory. You can't just assign a pointer to a char to a pointer to an int.
const char *asciiCString = [#"abc-zABC-Z" cString];
int cStringLen = [#"abc-zABC-Z" length];
for(i=0;i<cStringLen;i++) {
[asciiMArray addObject: [[NSNumber alloc] initWithInteger: asciiCString[i]]];
printf("%d\n",asciiCString[i]);
}
for(i=0;i<cStringLen;i++) {
NSLog(#"%#",[asciiMArray objectAtIndex: i]);
printf("%d\n",asciiCString[i]);
}
This is a code i wrote yesterday, to test some code of my learning face
It may look naive...but if it helps you....
asciiCString[i] returns you ASCII value of the char referenced at the index..
asciiMArray is a NSMutableArray Object